Signal system having improved oscilloscopic display means



Oct. 25, 1966 H. G. P. FORESTIER 3,231,841

SIGNAL SYSTEM HAVING IMPROVED OSCILLOSCOPIC DISPLAY MEANS Filed Nov. 25,1964 5 Sheets-Sheet 2 nine pulses eLqhlf pulses nine pulses eight pulsesA A A A E1 \f \f lllllll lllllll mum JL IL 1L JL 7 1 T0 T2 To 1- T0 T2T9 LF LY Lr LY' Henri G. P Foresfier INVENTOR Oct. 25, 1966 H. G. P.FORESTIER SIGNAL SYSTEM HAVING IMPROVED OSCILLOSCOPIC DISPLAY MEANSFiled NOV- 25, 1964 5 Sheets-Sheet 5 Henri GP Foresfier I'NVENTOR.

1966 H. s. P. FORESTIER 3,281,841

SIGNAL SYSTEM HAVING IMPROVED OSCILLOSCOPIC DISPLAY MEANS Filed NOV. 25,1964 5 Sheets-Sheet 4 28 V0 LTAGE GEARS I F44 PULSE GEN.

COMPARATOR VARIABLE 6P1 U RR 4 PHASE \r LOGIC C SHIFTER NTWK GP'Z PULSE\RADAR RCVR. PULSE GEN,

SPLITTER T CRT B 5 VOLTAGE CE CONTROL PULSE GEN 1 Lomc RT /1/" G52 urwx\RADAR PULSE GEN.) SPUTTER RcvR.

15 SAWTOOTH SINE-WAVE GEN. EA

KEY-PULSE GEN. CRT

Lro AND E2. EI|

KEY- PULSE GEN.

Henri GP Foresfier lNVENTOR r;

x {KM GENT Oct. 25, 1966 H. G. P. FORESTIER 3,281,341

SIGNAL SYSTEM HAVING IMPROVED OSCILLOSCOPIC DISPLAY MEANS Filed Nov. 25,1964 5 Sheets-Sheet 5 AGENT.

3,281,841 SIGNAL SYSTEM HAVING IMPROVED OSCILLO- SCOPIC DISPLAY MEANSHenri G. P. Forestier, Paris, France, assignor to Compagnie FrancaiseThomson-Houston, Paris, France, a French body corporate Filed Nov. 25,1964, Ser. No. 413,830 Claims priority, application France, Nov. 26,1963, 955,024 18 Claims. (Cl. 34313) This invention relates to systemsutilizing oscilloscopic, or cathode-ray tube, devices for the display ofrepetitive signals, such as echo signals received from a target in aradar system.

In an oscilloscopic device an electron beam is caused to sweep thefluorescent screen of the device by applying suitable cyclically varyingvoltages to the deflection electrode means of the device, such asvertical and horizontal electrostatic deflection plates. Concurrentlythe intensity of the beam is controlled by applying to a beam controlelectrode, such as a Wehnelt grid, the electric pulses or signals to bedisplayed. These signals are then displayed on the screen as brightspots. In many applications, owing to the cyclic character of thescanning curve or raster swept out by the electron beam across thescreen, coupled with the repetitive character of the signals displayed,the information conveyed by the positioning of the spots isnon-univocal, and hence ambiguous. Thus, in radar Work where the signalsto be displayed are echo signals reflected from a target in response toa continuous series of radar pulses transmitted towards the target, thedisplay may present a continuous series of spots aligned in an array onthe scanning raster, and indicating a series of target distances amongwhich the true distance is not easily seen, the display becomingambiguous. The need for such a repetitive and hence non-univocal displayarises in present-day radar techniques owning to the very great range oftarget distances which a given radar system is required to monitor.

In many cases the non-univocal character of the display is of nopractical consequence because the initial position of the target isknown and the target may thereafter be tracked throughout its subsequentdistance variations; that is, the information concerning true targetdistance is preserved through continuity. There are other cases,however, where the non-univocal character of the oscilloscopic displaycan lead to loss of information and consequent operational errors. Oneimportant such case occurs in connection with radar ranging systems ofthe socalled dual-transmission-rate type, In these systems, means areprovided for transmitting the radar signals at several differentrepetition rates, switch-over from one rate to another being effectedwhenever the target distance approaches a value which is a wholemultiple of the so-called critical distance associated with said onetransmission rate. This critical distance is defined as the minimumdistance at which echoes would be received at or about the same instantthat a subsequent radar signal is being transmitted, i.e. criticaldistance= /2cT, where c is the velocity of electromagnetic waves and Tis the reciprocal of the repetition rate or frequency of the transmittedradar pulses. When the target is situated at the critical distance or amultiple of it, the echoes tend to be confused or blanked out andoperation is unsatisfactory, hence the utility of the dual-rate systems.

An example of such as dual-rate radar system, in which the switch-overbetween transmission rates is effected automatically in response toactual target distance and at an optimum instant avoiding loss of signalat the time of switch-over, is disclosed in my earlier co-pendingapplication Ser. No. 337,352, filed on January 13, 1964.

In dual-rate radar systems of this general type the non- 3,281,841Patented Get. 25, 1966 univocal character of the target-distance displayhas been found to produce difliculties. After the target has beensuccessfully acquired and its true distance identified during theinitial, so-called acquisition stage of the ranging procedure, and thesystem has been set to its automatic target-tracking mode, theinformation concerning true distance is preserved through continuityonly so long as the transmission rate remains unaltered. However, at thefirst occurrence of automatic switch-over to the alternativetransmission rate due to the targets approaching a multiple of thecritical range, this information is liable to be lost and theacquisition procedure may then have to be performed all over again.

It is an object of this invention to provide a new type of oscilloscopicdisplay for repetitive signal systems which will completely overcome theaforementioned difficulties inherent in the non-univocal or repetitivecharacter of the display. Another object is to provide improved radioranging systems, especially dual-rate radar ranging systems, which willbe especially well-suited for the detection and ranging of remote,fast-moving targets, and will considerably simplify the proceduresinvolved as well as reduce operational errors. A more specific object isto provide an improved automatic dual-rate radar-ranging system of thetype disclosed and claimed in my co-pending patent applicationidentified above, modified with a view to simplifying inter alia theperformance of the target-acquisition stage of the procedure and toavoid loss of target during the subsequent, automatic tracking stage.

The above and further objects will appear as the disclosure proceeds.

In accordance with a basic aspect of the invention there is provided asignal system in which a first and a second signal train are produced ata first and a second repetition rate, respectively, said signal trainsalternating for equal periods of time at a third rate which is thegreatest common divisor of said first and second rates. Thus the signaltrains coincide at regular intervals. Said signal trains are applied toa beam-control electrode, e.g. Wehnelt grid, of a cathode-ray tube.There is applied to the deflection means, e.g. plates, of the tube acyclically varying voltage having a minor cycle period the reciprocal ofsaid first repetition rate and having a major cycle period thereciprocal of said third repetition rate. The electron beam of the tubeis thus made to sweep out a cyclic scanning curve (or raster) on thescreen of the device which cyclic curve has a line repetition frequencyequaling said first rate and has a scan repetition frequency equalingsaid third rate. Under these conditions, as will be shown hereinafter,the said signals will be displayed on the screen in alignment with twodifferent loci or curves during said alternating periods of time. Wherethe signals to be displayed are radar echo signals from a common targetor the like, the intersection of the two curves will provide anunambiguous indication of true target distance.

Exemplary embodiments of the invention will now be described withreference to the accompanying drawing. The description will makeespecial reference to cases where the invention is applied to adual-rate radar system of the kind disclosed in the earlier copendingapplication identified above, but it is to be understood that while thisdoes constitute at present a preferred application of the invention, theusefulness of the invention is by no means limited thereto. In thedrawing:

FIG. 1 is a functional diagram of part (the so-called keying section) ofa dual-rate phase-shift follow-up radar system of the general typedisclosed in my co-pending application, as modified in accordance withthis invention;

FIG. 2 is a pulse timing diagram illustrating the timing of the twoalternate trains of keying pulses and the uniform coincidence pulses,produced in the system of FIG. 1;

FIG. 3 is a diagram showing waveforms of beamdeflection volt-ages usedin the system of FIG. 1 to produce spiral scanning in accordance withthe invention;

FIG. 4 is a view of an oscilloscopic display provided in accordance withthe invention when using a spiral raster;

FIG. 5 is a functional diagram of the same system as the one shown inFIG. 1, partly illustrating the so-called follow-up section and otherdetails thereof;

FIG. 6 shows an osci-lloscopic display provided in accordance with theinvention when using a sinusoidal raster;

FIG. 7 is a partial diagram showing how the system of FIG. 1 may bemodified to provide the raster of FIG. 6;

FIG. 8 shows an oscilloscopic display provided in accordance with theinvention when using a parallel-line raster.

As indicated, the invention is applicable to a dualpulse-rate radartracking system provided with automatic switching means whereby theradar transmitter is switched from one keying frequency to anotherkeying frequency whenever the distance of a target being tracked entersa range in which the echo from it would be received at about the sameinstant a subsequent radar signal is being transmit-ted, were thetransmiter to continue sending at the former keying frequency.

A dual-rate radar system provided with automatic switching circuitry ofthis type was disclosed in my earlier co-pendin-g application Ser. No.337,352, filed January 13, 1964. As there disclosed, means are providedfor for generating two trains of keying pulses E1 and E2, at differentfrequencies so selected in relation to each other that the pulses in therespective trains intermittently coincide at periodic intervals. Thepulses of only one of the two trains, Ell or E2, are applied as keyingpulses to the radar transmitter at any given time in order to triggerthe transmission of the radar pulses towards the target. The switch-overfrom one to the other keying-pulse train is performed automaticallywhenever the target enters a distance range in which continuedtransmission at the former pulse rate would prove unsatisfactory owingto near-coincidence between transmitted pulses and received echoes.Furthermore, means are provided whereby the effective switching actionfrom the former keying-pulse train (say E1) to the other pulse train(say E2), will occur precisely at an instant at which the pulses of thetwo trains E1 and E2 coincide.

In other words, according to said co-pending application, whenever thetracking system of the radar senses that the target distance has entereda range of distances wherein continued transmission at the present pulserate (say F1) would lead to unsatisfactory operations, said systemissues a command signal, which states in efiect that prompt switchoverto a different transmission pulse rate (F2) is in order. However, suchswitchover does not occur immediately. Logical circuitry is providedwhereby the actual switchover to the alternative pulse trains (E2)awaits the occurrence of the next fol-lowing coincidence between the twopulse trains E1 and E2. At that instant, the keying-pulse train E1 isswitched off from the keying input of the radar transmitter and thealternative pulse train E2 is switched on instead.

In accordance with the present invention, the procedure just describedis modified, at least during the so-called target-acquisition stage ofthe radar tracking procedure, which is the preliminary stage of locatinga designated target and presetting the tracking follow-up section of theradar system to enable automatic tracking of the target during thesubsequent, tracking stage of said process.

During this preparatory acquisition stage, then, rather than switchingthe keying input to the transmitter between the two pulse trains E1 andE2 automatically in accordance with target distance as described above,the two pulse trains E1 and B2 are used in a. regular alternatingsequence, the switching between the two trains being constantly effectedevery time the two pulse trains coincide.

As shown in FIG. 1, GEl and GE2 represent the two keying-pulsegenerators similarly designated in my copending application. Keyinggenerator GE]. produces the keying-pulse train E1 at the first keyingfrequency or rate F1, and generator GEZ produces the keying-pulse trainE2 at the second keying frequency or rate F2. As described in thecopending application, the keying-pulse generators G131 and GEZ likewiseproduce, in the operation thereof, the two pulse trains Lrl and Lr2,which are at the same rates F1 and F2 as the respective keying-pulsetrains E1 and E2, but are somewhat broader than they.

The pulse trains Lrl and Lr2 are applied to an AND- gate 9. The outputof this AND-gate, therefore, is a pulse train Lrll, which is at thefrequency F0, the greatest common divisor of F1 and F2. In other words,a pulse Lrtl appears at the output of AND-gate 9 every time the twopulse trains Lrl and Lr2 (or the two .pulse trains El and E2) coincide.The pulse train Lr0 from AND-gate 9 is applied (through a switch 32closed at this time, and an OR-gate 34, which will both be laterreferred to in detail) to the common input of a bistable element 12, soas to switch it alternately between two states in each of which arespective one of its two outputs produces a voltage output. The outputsof the bistable element 12 are applied to respective AND-gates 13 and 13whose other inputs receive the keying-pulse trains E1 and E2respectively. The outputs from both AND-gates 13 and 13 are applied toan OR-gate 14 the output from which is applied to the keying input ofradar transmitter RT. The components described above are contained in alogical network generally deseignated LN.

It will be evident that during those periods when bistable circuit 12has its upper output energized the keying input to transmitter Treceives E1 pulses (at repetition rate F1) from keying-pulse generatorGEl, while during those alternate periods when bistable circuit 12 hasits lower output energized the keying input to transmitter RT receivesE2 pulses (at rate F2) from key-pulse generator GEZ. Since bistableelement 12 is switched between its two states in step with the Lrt)pulses at the rate F0, which is the greatest common divisior of thepulse rates F1 and F2, it is seen that every time an LrO pulse occurs,the keying frequency applied to the radar transmitter RT is switchedfrom value F1 to F2, or from F2 to F1, as the case may be.

As a result of this action the sequence of keying pulses applied totransmitter RT proceeds according to the time pattern shown in FIG. 2.In this figure it has been assumed that the alternate repetitionfrequencies F1 and F2 used are in the ratio Fl/F2=9/ 8. In the otherwords, the following relationships exist between the two keyingpulserates F1 and F2 and the pulse rate F0 of the pulses Lr0 from AND-gate 9:

F0, as already indicated, being the greatest common divisor of F1 andF2.

The above assumption is in agreement with the conditions in one actualembodiment of the invention, wherein the following values wereapproximately used: F1=576; F2:512; hence F0=64.

The lower line in FIG. 2 indicates the Lr0 pulses produced from AND-gate9, which are separated in time by equal intervals Ti) (with T 0:1/F0).The upper line in the figure indicates the keying pulses applied toradar transmitter RT. As shown, during a first T0 period the pulsesapplied by logic network LN to the transmitter are the E1 pulses at therate F1. The LrO pulse appearing at the end of this first T0 period actsto switch the output from the pulse train E1 to the pulse E2 at therepetition rate F2. Thus during the second T0 period E2 pulses atrepetition rate F2 are applied to the keying input of trans mitter RT.At the termination of this second T0 period the Lr0 pulse occurring atthe output from AND-gate 9 acts to switch the logic-network output backfrom pulse train E2 to pulse train E1; and so on repeatedly. It willthen be evident with the above numerical assumptions, that during oneset of alternate T0 periods (herein the odd T0 periods) the radartransmitter will receive at its keying input a series of nine E1 pulsesat the higher repetition frequency F1 and during the other set ofalternate T0 periods (herein the even-numbered set) the transmitter willreceive a series of eight E2 pulses at the lower repetition frequencyF2. The period T2 separating a pair of the E2 pulses is, of course 9/8the period T1 separating a pair of the E1 pulses.

In the numerical example referred to above, where Fl=576, F2:5l2 andF0=64, the time periods T0, T1 and T2 have the following values inmilliseconds:

TO=15.6255 ms.; Tl=1.736 ms.; T2=l.953 ms.

Returning to FIG. 1, a cathode-ray tube CRT is schematically shown forthe display of the radar signals. The tube CRT includes a controlelectrode, e.g. a Wehnelt grid, schematically indicated at CE, a pair ofvertical deflection electrodes VD and a pair of horizontal deflectionelectrodes HD. Video signals from the radar receiver RR are applied in aconventional manner to the control electrode CE by way of connectionthrough an amplifier EA. In order to display these video (echo) signalsupon the screen of the cathode-ray tube, the voltages applied to thevertical and horizontal deflection electrodes VD and CD are controlledin accordance with this invention in the manner now to be described.

There is provided a vertical deflection modulator VM the output fromwhich is applied by way of an amplifier VA to the vertical-deflectionplates VD; and there is provided a horizontal-deflection modulator HMthe output from which is applied by way of an amplifier HA to thehorizontal-deflection plates HD. The deflection modulators VM and HMeach have a carrier input 16 and 17 respectively, which are bothconnected to receive the E1 pulses from keying generator GE1. It isnoted however that the carrier input 16 to vertical-deflection modulatorVM has a delay circuit 18 interposed therein which imparts a 90 delay tothe E1 pulses applied to that modulator with respect to the E1 pulsesapplied to horizontal modulator HM.

Each of the deflection modulators VM, HM further has a modulating inputconnected to the output of a sawtooth generator SG. Generator SG has acontrol input 20 connected to receive Lr0 pulses from the output of AND-gate 9, so as to emit a linear sawtooth wave at each occurrence of anLr0 pulse.

With the arrangement described, the voltages applied by amplifiers VAand HA to the vertical and horizontal deflection plates VD and HD of thecathode-ray tube will have the waveforms shown at UV and UH respectivelyin FIG. 3.

It will be seen that the horizontal deflection voltage UH (upper curve)is a waveform composed of a series of sawtooth cycles each having aduration T 0=1/F0 since F0 is the modulating-envelope frequency appliedby sawtooth generator SG to the modulating input of modulator HM. Eachsawtooth cycle is composed of a pseudo-sine-curve of linearly increasingamplitude with the period of each pseudosinusoid being given by Tl=1/Flsince F1 is the carrier frequency applied to input 17 of modulator HM.The vertical deflection voltage UV (lower curve) is a waveform ofidentical shape but displaced in time one fourth the minor period T1with respect to the horizontal deflection voltage UH.

Under these conditions the trace of the electron beam in the cathode-raytube CRT when subjected to the joint action of the two deflectionvoltages UV and UH will sweep out on the screen of the tube anArchimedean spiral S as indicated in FIG. 4. Each scanning cycle, i.e.the time period required for the spot to sweep out the completemulti-turn spiral arc and return to its initial point 0 is the majorperiod T0=1/F0 (about 15.6 milliseconds in the above-mentioned example).The pitch (or line repetition period) of the spiral, i.e. the timeperiod required for the spot to travel from any point of the spiral tothe corresponding point on the immediately adjacent turn of the spiralsituated on the same radial line such as OR, is the minor period Tl=1/Fl(about 1.7 ms. in the practical example).

It will be understood that the spiral scanning pattern just described,wherein the scanning cycle frequency is F0 and the line repetition orpitch frequency is F1, is used according to the invention both when theradar system is transmitting at the repetition rate F1 (keying pulsesE1) and when transmitting at the repetition rate F2 (keying pulses E2).Consider first those periods, the odd-numbered T0 periods in FIG. 2,when the system transmitter is using the keying pulses E1 (repetitionrate F1). Then successive echo signals received from the target (assumedto be stationary) will appear as luminous spots a, a, a" etc. on thespiral raster S displayed on the radar screen, all aligned along acommon radius such as 0R. This is evident when one considers that aseach spot on the screen is produced by a video signal applied to controlelectrode CE over an amplifier EA connected to the receiver output 15,when the target is stationary these video signals occur with the samerepetition frequency as the transmission repetition frequency F1.

However, during those periods, i.e. the even-numbered T0 periods in FIG.2, when the system transmitter is using the keying pulses E2 at therepetition rate F2, then the video signals are applied through amplifierEA with this same repetition rate F2, different from the scanningfrequency which is still F1, so that the spots b, b, b etc. indicativeof successive echoes from the target will now line up along a spiral are2, which differs from the scanning spiral S.

This can be established mathematically as follows.

Let us first determine the polar equation of the scanning spiral S sweptby the electron beam during each scan cycle T0. The equations for thehorizontal and vertical deflecting voltages (FIG. 3) can be respectivelywritten as follows:

t y UV: (rO-I-H cos t T-1*O+H- -TO+H-FO-t Eliminating the parameter t:

H T1 770+Z'fi6 Equation 4 is the equation of an Archimedean spiral S inwhich the spiral pitch, i.e. the radial distance between correspondingpoints of adjacent turns such as the points a, (1, etc. marked out onradius OR in FIG. 3, is (T 1/TO)H. During those periods when the systemis operating at keying frequency F1 so that consecutive echo signalsfrom a stationary target are spaced apart by the constant period T1, ifa first of these echoes occurs at a 7 time 1'1 it will be displayed as aspot (a) having the polar angle 1=21rt1/ T 1, and the next followingecho will occur at the time t1=t'1+T1 and will be displayed as a spot(a') having the polar angle The two polar angles are seen to differ bythe quantity 010'l=21r, and hence the two consecutive echoes arepositioned on a common radius O-R, as earlier stated.

Next consider the case where the transmitter is using the key pulses E2at the repetition frequency F2, i.e. during the even-numbered T0 periodsin FIG. 2. The consecutive echoes from the stationary target are nowarriving at intervals of T2 (wherein T2 T1). A first of these echoesappearing at a time t'2 produces a spot on the scanning spiral S say atb, having the polar angle The next echo occurs at time (t'Z-l-T 2) andproduces a spot b having the polar angle t 2 T2 T1 The differencebetween the two polar angles as The corresponding difference between thevector radii of the two points b, b, as deduced from the second Equation3 is Ar=H(T2/TO).

The above values of A0 and Ar remain constant when applied to any twoconsecutive points such as b and b; b and b; and so forth. Hence thesepoints are all laid out along a spiral 2 whose equation can readily beobtained by noting that the dilference Aqb between the polar angles ofany two consecutive points as measured along the spiral 2 is equal tothe difference A0 between the polar angles of the same two points asmeasured along the spiral S, minus 360". That is Spiral 2 is thereforesuch that whenever the polar angle is increased by the vector radius ofthe spiral 2 increases by A =H T 2/ T0) The equation for such a spiral,as can be instantly verified, must be of the form l 2M0 T2- T1) This canbe rewritten P=Z fi (o) or, putting Fl /F0=a and F2/F0=b,

H 1 a m (6) In the numerical example considered above, where a=9 andb=8, Equation 6 becomes H P' (s o) (7) Such is the equation of thisspiral 2 shown in FIG. 4.

In the above Equations 5, 6 and 7, the quantity 4m depends on the targetdistance.

The principal results of the above mathematical analysis can besummarized as follows:

When in accordance with this invention two different keying repetitionfrequencies (F1, F2) are used alternately for radar transmission and aspiral scanning raster is used for displaying the echo signals on theradar screen,

8 said spiral raster having a line repetition frequency constantly equalto one (P1) of said alternate keying frequencies and a scan recurrencefrequency constantly equal to the greatest common divisor (P0) of bothkeying frequencies used, then:

(1) during those periods when said .one keying frequency (F1) is beingused for transmission, the echo signals from a stationary target allline up on a common radial line; and

(2) During those alternate periods when the other keying frequency (F2)is being used for transmission, the echo signals from the stationarytarget all line up on a spiral diiferent from (and radially expandedwith respect to) the scanning spiral at the intersections of said secondspiral with the successive turns of the first, or scanning, spiral.

At each instant of switchover from one to the other of the two keyingfrequencies F1 and F2, i.e. at the termination of each of the successiveT0 periods (see FIG. 2), the position of the spot formed by the echofrom the common target must necessarily be the same whether given by theone or the other keying frequency. Hence at each instant of switchoverthe echo spot is positioned at the intersection of radius OR (the locusof spots given by the keying frequency F1) and spiral Z (the locus ofspots as given by keying frequency F2). This partic ular spot,therefore, appears as a double spot, i.e. a spot of enlarged diameter,as indicated at A, and is thus readily distinguishable from theneighboring spots.

In this manner the ambiguity which would otherwise be present as to theparticular spot representing actual target position is avoided.

It will be understood that the type of ambiguity which the system of theinvention serves to avoid is essentially due to the inherentlyrepetitive character of the scanning curve and of the signals. Thus, ifwe consider a radar system transmitting continuously at a keyingrepetition frequency of F1, and a display using a spiral scanning rasterat the same line repetition frequency of F1 so as to provide the spiralS as in FIG. 4, then a given target at a distance of x kilometers willbe displayed as an array of spots corresponding to distances differingfrom one another by quantities of /2 c/Fl kilometers, that is, in theabove example, quantities of 260.4 kilometers. From an observation ofthe radar screen it will therefore be impossible to say whether thetarget distance is x, or xi260.4, or xi520.8, or xi78l.2 km., etc. Whensuch a scanning scheme is used with a single-frequency radar system theabove uncertainty is, usually, of no consequence, because once theautomatic tracking follow-up section of the radar system has been setmanually to track a designated target (e.g. by the phase-shift follow-upmethod described in the copending application referred to above), itwill continue automatically to keep track, there being no reason for thesystem to lose track of the target at any time. On the other hand, inthe case of a system using several different transmission keyingfrequencies, the automatic tracking section of the system would tend tolose track of the target at the instant of switchover from oneparticular keying-pulse train i.e. the one used at the time of themanual setting of said tracking section, to the other keying-pulse trainshould the target enter a distance range requiring such switchover asexplained in the copending application. The display system describedherein was primarily designed to overcome this difliculty.

It will be understood from the foregoing that during the initial,so-called acquisition stage of operation of the improved radar system,in which stage the transmission keying rate is continually alternatingbetween the two values F1 and F2 at intervals of T0 as described aboveand shown in FIG. 2, the true distance of the target is made knownunambiguously to the radar operator as the particular distance indicatedby the enlarged (doublepoint) spot A, and no other of the radial arrayof spots 9 a, a, a", appearing on the screen. The operator can thenproceed to set the tracking section of the system manually so that itwill track the unambiguously determined target, this manual settingoperation being accomplished through the following means forming part ofthis invention.

The control electrode CE of the cathode ray tube CRT (see FIG. 5) hasapplied to it over a connection 22a socalled marker pulse which is apulse recurring at the repetition frequency F0, the same as therepetition frequency of the Lri) pulses mentioned above. These markerpulses however are derived not from the AND gate 9 producing said Lrtlpulses, but from a similar AND gate (not shown) forming part of theselector logic network L in the follow-up section of the system, hereschematically shown. As fully disclosed in the afore-mentioned copendingapplication, the said logic network L produces a train of pulses calledL'c in said application, which repeat at the repetition rate F0, i.e.the greatest common divisor of the keying rates F1 and F2, but which arevariably delayed with respect to the similar pulses (herein called Lrtl)produced in the keying section of the radar system. The delay isproduced through the action of a variable phase shifter schematicallyshown herein and fully described in the copending earlier application.In accordance with the present invention, the said delayed pulses L'c atthe repetition rate F are used to provide marker pulses for the display,and are for this purpose passed through a marker-pulse amplifier MA andthrough an elongator-and-splitter circuit PS, of any suitableconventional design, such that the output of the circuit PS, whenapplied by line 22 through amplifier MA to the control electrode CE ofthe cathode-ray tube, will appear on the screen as a split, elongatedstreak or marker index shown at MI in FIG. 4. Since the marker pulserecurs at repetition rate Ft), the resulting marker index MI isdisplayed at a particular position along the spiral S and thisparticular position is determined by the variable delay imparted to theL'c pulses by the variable phase shifter g5. Thus, the radar operator,through selective adjustment of the phase shifter 4 is able to shift themarker index MI along the sweep spiral S until it frames or straddlesthe spot A indicative of true target distance, as indicated in dottedlines at MI in FIG. 4. To enable this adjustment, the tracking follow-upservo-motor M (FIG.

), which as described in the copending application normally serves tooperate the phase shifter under control of the error voltage suppliedfrom time-comparator device C, is here shown as beiing manuallyoperable. For this purpose a reverser switch 26 is interposed in theconnection 24- from comparator C to motor M. The switch 26 when movedfrom its full-line position (the normal position during automatictracking mode of operation of the system) to its dotted-line positionduring the initial acquisition mode of system operation, connects themotor input to an auxiliary voltage source AS through a potentiometer28. In this position of the switch 26, manual operation of thepotentiometer control knob 39 will actuate motor M to modify, throughgearbox B, the mechanical setting of phase shifter 1; and thereby shiftthe marker index MI along the spiral scan curve S on the radar screen.After the operator has rotated knob 36 so that the marker index MI hasframed the true-distance spot A as shown at MI, he moves the switch 26to its automatic-tracking position shown in full lines. At this time thephase shift imparted by the phase shifter to the L'c pulses, andsimultaneously imparted thereby to the follow-up pulse trains P1 and P2generated by the follow-up pulse generators GP1 and GPZ, exactly equalsthe time required by the radar signals to travel from transmitter RT tothe target and for the echo (or response) signals to travel back fromthe target to receiver RR. Hence, as explained in the copendingapplication, the follow-up pulses P or P, as the case may be, applied byfollow-up logic L to comparator C coincide in time with the echo signalsapplied to the comparator from receiver RR, and so long as thiscondition obtains the follow-up servomotor M remains stationary. In caseof a discrepancy between the comparator inputs due to target movement orsystem disturbance, the comparator applies an output voltage to motor Mcausing the motor to readjust the setting of phase shifter until a newequilibrium position has been reached. Automatic tracking of the targetthus proceeds generally as described in the copending application.

It will be understood that during this automatic tracking stage of theprocedure, the fixedly alternating switching action between thekeying-pulse trains E1 and E2 at the repetition frequencies F1 and F2,as explained above with reference to FIG. 2 and as used in the initialacquisition stage, is no longer effected. Instead, the switching betweenthe two keying frequencies is now automatically commenced under controlof target distance in the manner disclosed in the copending application.That is, keying-pulse trains at one rate, say F1, would continue to beapplied by network LN to the keying input of transmitter RT (andfollow-up pulse trains P1 at the same rate would continue to be appliedby follow-up network L to comparator C) for as long as the target doesnot approach a distance which is an integral multiple of the criticaldistance c/2F1, at which time the readings would become unreliable. Whenthe target does enter such a critical distance range, the keyingfrequency is switched to the other of the two available values, F2, bothin the keying section and in the follow-up section of the system, all asexplained in the copending application.

In this connection it is important to note that the logic network heredesignated LN and described with reference to FIG. 1 as performing abasic function of the present invention, namely the continual,uniformly-timed switching between the alternate pulse trains E1 and E2at equal T0 intervals, has considerable circuitry in common with theselector logic network called L in the copending application and shownin detail in FIG. 4 of that application. For convenience, the elementscommon to the two networks are designated by the same numerals in bothfigures.

In actual practice, in cases where the present invention is applied to aradar system of the type disclosed in the prior application and asdescribed above, there would be used a common logical network capable ofbeing switched between the two circuit conditions respectively shown inFIG. 1 of the present application and FIG. 4 of the prior one.

Accordingly, the binary circuit 12 is shown in FIG. 1 as having itsinput alternatively controllable, by way of OR-gate 34 earlier referredto, through the pair of automatic-control connections 36, 38 which maybe regarded as connected to the outputs of the respective AND gatescalled 11 and H in FIG. 4 of the copending application. When theswitches 40 interposed in the connections 36 and 33 are open, and theswitch 32 interposed in the connection from AND gate 9 to OR gate 34 isclosed, then the logic network LN operates, in the manner describedabove in detail, to cause regular alternation between the two states ofbinary circuit 12, and hence between the two pulse trains E1 and E2applied to the keying input of transmitter RT, at the rate of the L0pulses. When on the other hand switch 32 is open and switches 40 areclosed, the binary circuit 12 is switched between its two states independency on target distance, and the switching between the twokeying-pulse trains E1 and E2 is similarly made dependent on targetdistance, in accordance with the operating mode described in theco-pending application.

The switches 32 and 49 here shown for clarity as separate, gangedswitches may in actual practice be replaced by a single switchperforming an equivalent function. The operation of the switching means32-40 is preferably ganged with the operation of the manual switch 26(FIG.

), as indicated by the mechanical connection 44 shown in both FIGS. 1and 5, so that the radar operator, on moving switch 26 from itsfull-line position to its dottedline position on completion of thetarget-acquisition stage of the procedure described above,simultaneously switches the network LN from the circuit condition shownin FIG. 1 hereof to a condition similar to that shown in FIG. 4 of theearlier copending application, in order to provide for automaticinterchange between the keyingpulse trains under control of targetdistance during the automatic tracking stage of the procedure.

In FIG. 5, the deflection-voltage control means shown in detail in FIG.1 and described in connection with that figure are schematically shownas the block DC.

As stated earlier, in a display system according to this invention theecho spot indicative of the true distance of a target appears in theform of an enlarged-diameter spot A by virtue of being the geometricintersection of the two echo loci OR and 2. While such anenlarged-diameter double spot will generally be found sufficient topermit the radar operator to pick out the true echo without anyhesitation from among the neighboring spots of lesser diameter and/ orlesser brightness, the invention contemplates the provision of means forrendering such recognition even easier and more positive.

As shown in FIG. 1, the lower output of binary element 12, i.e., thatoutput thereof which is energized during those alternate Ti} periodswhen E2 keying pulses are being utilized, is applied to the input of aringing oscillator Rt). When energized, oscillator R0 produces asmall-amplitude output wave at a frequency which preferably isapproximately the reciprocal of the pulse width of the radar pulses usedin the system. The output from oscillator R0 is connected to the outputof vertical-deflection amplifier VA through a 90-ph ase shifter or delaydevice, and is applied to the output of horizontal-deflection amplifierI-IA directly.

As a result of this arrangement, during those alternate T0 periods whenthe keying pulses E2 at repetition rate F2 are being used to key thetransmitter RT, a smallamplitude, high-frequency wobble voltage,produced by ringing oscillator R0, is superimposed over the normalspiral deflecting voltages applied to the vertical and horizontaldeflector plates of the cathode-ray tube, so that the echo spotsdisplayed during those T0 periods, which as earlier described arearrayed along the spinal 2, appear as open circles rather than points(see FIG. 4). The particular echo which lies on the intersection withradius OR and hence represents true target distance, appears as a dottedcircle and thus provides a clear and unmistaketable indication of truetarget distance.

The improved radar scanning and display system of the invention issusceptible of a number of embodiments and modifications other thanthose disclosed. One especially important class of modificationsinvolves the type of scanning raster used, which may be other than thespiral ras-ters heretofore considered. As one example of such amodification, FIG. 6 illustrates the type of display obtained with thesystem of the invention when using a sinusoidal rather than a spiralraster. Here the cyclic sweeping or scanning curve 5, instead of beingan Archimedean spiral as the similarly designated curve in FIG. 4, is asine-curve. As before, the repetition frequency of this curve, i.e. thefrequency at which the entire curve S is repeatedly swept out by theelectron beam across the cathode-ray screen, is F0, the greatest commondivisor of the two keying pulse rates F1 and F2, while the dinerepetition frequency, i.e., the frequency at which corresponding pointssuch as a, a, a" of consecutive sine cycles are scanned by the beam, inone of the two keying frequencies used, herein F1.

A sinusoidal scanning raster of this kind can be obtained by the meansshown in the partial diagram of FIG. 7 in which components correspondingto components presentin FIG. 1 are similarly designated. As shown,

t2 the horizontal-deflection amplifier HA is fed with a sinewave voltageat the frequency Fl, supplied by sine-wave generator SW whose inputreceives the E1 pulses from keying-pulse generator GEll. Thevertical-deflection amplifier VA is fed with a linear sawtooth wavehaving the cycle repetition frequency F0, the greatest common divisor ofthe keying frequencies F1 and F2, as derived from sawtooth generator SGhaving its input connected to the output of AND-gate 9.

With such an arrangement, the equations for the horizontal and verticaldeflection voltages can be written as follows:

L 1/ l/o (8) Equations 8 are the parametric Cartesian equations for thescanning curve S. The quantities x and y are constants of the sawtoothgenerator SG and sinewave generator SW and define the boundaries of thescanning raster as shown in FIG. 6.

In those alternate T0 periods when the keying pulses at frequency F1 arebeing used, it is obvious that the echo spots a, a, a, a' are allaligned on a common line RR parallel to the y axis as shown. In thoseother alternate Ttl periods when the keying pulses at frequency F2 arebeing used, it can be shown that the echo spots b, b, b", b', are allarrayed on a sine curve 2 the Cartesian equation of which can be written2 l 5 x sin 1r(a b)y (9) where a and b have the meanings .precedinglygiven and no is determined by the actual target distance. When targetdistance varies the sine curve 2 is bodily displaced parallel to the ycoordinate axis, just as in FIG. 4 the spiral E was in the samecircumstances bodily rotated. It will be noted that the diflerence (a-b)between the ratios of the respective keying frequencies to their commondivisor frequency, F0, is according to the invention preferably selectedequal to unity, as in the numerical example previously considered wherea=9 and 12:8, in order that the differential sweep curve herein called21 shall present a minimum number of intersections with any line such asO-R (FIG. 4) or R'-R (FIG. 6) over the extent of the major period T0.

The above results can be easily generalized to the broad case of aperiodic curve of any character whatever being used as the cyclicscanning raster, provided the line repetition frequency of the curve ismade equal to one (Fl) of the alternate keying frequencies used forradar transmission according to the invention, and the scan recurrencefrequency of the curve is made equal to the other (F2) of said alternatekeying frequencies. It can then be stated, for this broad case, that:

(1) During those periods when one keying frequency (F1) is being usedfor transmission, the echo signals from a stationary target all line upon a common line parallel to one coordinate (the repetitive coordinate)axis of the scanning raster; while (2) During those alternate periodswhen the other keying frequency (F2) is being used, the echo signals areall arrayed upon a differential curve of similar nature to that of thescanning curve, but expanded with respect to the scanning curve in adirection parallel to the aforementioned repetitive coordinate axis, atthe intersections of said second curve with the firs-t, i.e. scanningcurve; and consequently (3) The unique, or quasi-unique, intersection ofthe expanded differential curve with the said common line parallel tothe repetitive coordinate taxis will provide an unambiguous indicationof the ture target distance.

As an additional example of a type of oscillographic display with whichthe invention is usable, FIG. 8 shows a parallel-line scanning curve orraster. In this embodiment the scanning curve swept out by the electronbeam at the scan recurrence rate F6 is a family of parallel lines S,inclined to the x-axis of the oscilloscope screen. The line repetitionfrequency is the keying rate F1. During those T periods where thekeying-pulse rate F1 is being used, the echo spots are aligned as at a,a, a along a line R'R parallel to the y axis. During the alternate Tl)periods when the pulse rate F2 is used, the echo spots are aligned as atb, b, b" along a line 2. The intersection A of 2 with R-R provides theunique indication of true target distance. The means for developingdeflection voltages capable of providing a display of the type shown inFIG. 8 will be readily conceived by those familiar with the art from theexplanations previously given herein.

Clearly, in each of the modified embodiments of the invention shown inFIGS. 6, 7 and in FIG. 8, wobble.- voltage means similar to thosedescribed in connection with the first embodiment may be provided inorder to display circles around the spots of one of the arrays, if thisis desired.

It should be understood that the novel method of oscilloscopic displayprovided in accordance with the invention, while being of considerablevalue when applied to dual-rate radar systems such as the automaticdual-rate, phase-shift follow-up system specifically described herein asan exemplary embodiment of the invention, is susceptible of many otherapplications, such as in monitoring a chain of separate radar stationstransmitting at different frequencies, as well as in other cases whererepetitive signals are to be displayed. While in the embodimentsdescribed both pulse rates F1 and F2 were inherently available in thesignal system with which the oscilloscopic display systems isassociated, it should be understood that in other applications of theinvention only one such pulse rate (say F1) may be present in the signalsystem, in which case the companion pulse rate (F2), as well as thecoincidence pulse rate (F0) would be developed especially for thepurposes of the invention.

It should also be observed that whereas in the examples here disclosedthe higher of the two pulse rates, termed F1, was used as the linerepetition rate of the scanning curves S, this is not essential, sincethe lower of the two pulse rates may Well be used for that purpose. Theonly difference would be that the differential curve 2 would be reversedin position with respect to the positions shown.

What I claim is:

1. A signal system comprising means for producing a first signal trainat a first repetition rate;

means for producing a second signal train at a second repetition rate;

means for alternately enabling said signal trains for equal periods oftime at a third repetition rate which is the greatest common divisor ofsaid first and second rates;

an oscilloscopic display device including electron-beamproducing means,deflection electrode means, beamcontrol electrode means and a screen;means for applying said first and second signal trains to the controlelectrode means of the display device;

means for developing a cyclically varying voltage having a minor cycleperiod the reciprocal of said first repetition rate and a major cycleperiod the reciprocal of said third repetition rate; and

means for applying said voltage to the deflection electrode meanswhereby said beam will sweep out on the screen a cyclic scanning curvehaving a line repetition frequency equaling said first rate and having ascan repetition frequency equaling said third rate, and

whereby said first signals will be displayed on a linear first locuscomposed of the corresponding points of successive cycles of said cyclescanning curve while said second signals will be displayed on a secondlocus comprising another curve intersecting said linear locus.

2. The system claimed in claim 1, including means operative during thosealternate periods of time when said second signals are produced forapplying to said deflection electrode means a relatively high-frequency,small-amplitude additional voltage whereby to dilferentiate the displayof said second signals from the display of said first signals.

3. A signal system comprising means for producing a first signal trainat a first repetition rate;

means for producing a second signal train at a second repetition rate;

means for alternately enabling said signal trains for equal periods oftime at a third repetition rate the greatest common divisor of saidfirst and second rates;

an oscilloscopic display device including electron-beamproducing means,deflection electrode means, beamcontrol electrode means and a screen;means for applying said first and second signal trains to the controlelectrode means of the display device;

means for developing cyclically varying spiral-deflection voltageshaving a minor cycle period the reciprocal of said first repetition rateand a major cycle period the reciprocal of said third repetition rate;and means for applying said voltages to the deflection electrode meanswhereby said beam will sweep out on the screen a cyclic spiral scanningcurve having a line repetition frequency equalling said first rate andhaving a scan repetion frequency equalling said third rate, and 7whereby said first signals will be displayed as the intersections ofsuccessive turns of said spiral scanning curve with a common radial lineof the screen while said second signals will be displayed as theintersections of successive turns of said spiral scanning curve withanother spiral curve expanded with respect to the scanning spiral.

4. The system claimed in claim 1, wherein said cyclically varyingvoltage includes a vertical and a horizontal deflection-voltagecomponent, said voltage components being cyclically varying inaccordance with said minor and major cycles respectively, where-by saidscanning curve is a curve cyclically repetitive along one Cartesiancoordinate, said first locus is a line parallel to said one repetitivecoordinate and said second locus comprises another curve generallysimilar to said scanning curve but expanded with respect thereto in adirection parallel to said one coordinate.

5'. The system claimed in claim 1, including means for applying to saiddeflection electrode means a marker signal train at a repetition rateequaling said third rate, thereby to display said marker signal train asa marker index on the screen of the device, and means for adjusting thetime relationship of said marker signal train with respect to said firstand second signal trains whereby said marker index may be brought intocoincidence with the intersection of said loci.

6. A signal system comprising means for generating a first train ofcontrol signals at a first repetition rate;

means for generating a second .train of control signals at a secondrepetition rate; means for deriving from said first and second signaltrains a third train of signals at a third repetition rate the greatestcommon division of said first two rates;

means controlled by said third signal train for automatically switchingbetween said first two signal trains whereby said signal trains areenabled during respective periods of time alternating at said thirdrepetition rate;

means for deriving from said first and second controlsignal trains afirst and a second train of resulting signals having the same repetitionrates as said first and second control-signal trains respectively buttime displaced with respect thereto;

an oscilloscopic display device including electron-beamproducing means,deflection electrode means, control electrode means and a screen;

means for applying said resulting-signal trains to said controlelectrode means of said device for display on said screen; means fordeveloping at least one cyclically varying voltage having a minor cycleperiod the reciprocal of said first repetition rate and a major cycleperiod the reciprocal of said third repetition rate; and

means for applying said voltage to the electrode means whereby said beamwill sweep out on the screen a cyclic scanning curve having a linerepetition frequency equaling said first rate and a scan repetitionfrequency equaling said third rate, and

whereby said resulting signals will be displayed on the screen at theintersections of said scanning curve with a line indicative of said timedisplacement during periods when said first signal trains is enabled andat the intersections of said scanning curve with another curve duringperiods when said second signal train is enabled and the intersectionbetween said line and said other curve will provide an unambiguousidentification of said time displacement.

7. The system defined in claim 6 including means operative when one ofsaid first and second signal trains is enabled for applying to thedeflection electrode means a high-frequency small-amplitude additionalvoltage whereby to differentiate the display of said second signals fromthe display of said first signals.

8. The system defined in claim 6, including means for varying saidvoltage to control the beam for sweeping out a spiral curve having aline repetition frequency equaling said first rate and a scan repetitionfrequency equaling said third rate, whereby said line is a radial lineand said other curve is a spiral expanded with respect to said firstspiral.

9. The system defined in claim 6, including means for varying saidvoltage to control the beam for sweeping out a scanning curve which isperiodically repetitive along one Cartesian coordinate of the screenwhereby said line is parallel to the repetitive coordinate and saidother curve is a curve similar to but expanded with respect to saidscanning curve.

19. A radio ranging system comprising a transmitter having a keyinginput for controlling the repetition rate of signals transmitted therebytoward a target;

a receiver for receiving response signals from the tarmeans forgenerating a first train of keying pulses at a first rate;

means for generating a second trains of keying pulses at a second rate;

coincidence means deriving from said first and second pulse trains athird pulse train at a third rate the greatest common divisor of saidfirst and second rates;

selector logic means having inputs connected to saidkeying-pulse-generating means and output connected to said keying inputof the transmitter;

said logic means further having a controlling input connected to saidcoincidence means whereby to apply said first and second keying-pulsetrains to said keying input for respective periods regularly alternatingin accordance with said third rate;

an oscillographic dispsay device including ele:tron-beamproducing means,deflection electrode means, beamcontrol electrode means and a screen;

means connecting said receiver to said control electrode means to applysaid response signals thereto for display on said screen;

voltage-developing means having inputs connected to said firstkeying-pulse generating means and said coincidence means for developingat least one cyclically varying voltage having a minor cycle period thereciprocal of said first repetition rate and a major cycle period thereciprocal of said third rate;

said voltage-developing means having a voltage output connected to saiddeflection electrode means of the display device whereby said electronbeam will sweep out on the screen a cyclic scanning curve having a linerepetition frequency equaling said first rate and a scan repetitionfrequency equaling said third rate, and

whereby said response signals will be displayed on the screen at theintersections of said scanning curve with a line indicative of targetdistance during periods when said first keying pulses are applied and atthe intersections of said scanning curve with a different curve duringperiods when said second keying pulse train is applied, and theintersection between said line and said different curve will provide anunambiguous identification of target distance.

11. The system claimed in claim 10, including control means connected tosaid receiver responsive to target distance, and said logic means havinga second controlling input connectable to said control means forselectively applying said first and second keying-pulse trains to thekeying input of the transmitter in dependency on target distance, andoperator-controlled switching means displaceable between a firstposition in which said first controlling input is connected to the logicmeans and a second position in which said second controlling input isconnected to said logic means.

12. The system claimed in claim 10, including means for applying to saiddeflection electrode means a marker pulse train at a repetition rateequaling said third rate, thereby to display said marker pulse train asa marker index on the screen of the device, and means for adjusting thetime relationship of said marker pulse train with respect to said firstand second pulse trains whereby said marker index may be brought intocoincidence with said intersection.

13. The system claimed in claim 12, further including anautomatic-tracking follow-up section connected to said receiver, saidmarker pulse train being generated in said follow-up section, afollow-up motor in said follow-up section movable to track a target andsimultaneously alter the time relationship of said marker pulse train,and an energizing input for said motor, said operator-controlledswitching means when displaced to said first position simultaneouslyconnecting said motor input to adjustable voltage means for adjustablyaltering the time relationship of said marker pulse train until saidmarker index has been brought into coincidence with said intersectionbetween said line and said different curve, said switching means whendisplaced to said second position connecting said motor input in aservo-loop with said receiver for automatically tracking the target.

14. The system claimed in claim 19, including means connected foroperation by said logic means for applying during the alternate periodsof application of said second keying-pulse train a relativelyhigh-frequency, smallamplitude additional voltage to said deflectingelectrode means whereby to display small circles around theintersections of said scanning curve with said different curve.

15. The system claimed in claim 10, comprising means for varying saidvoltage to control the beam for sweeping out a first spiral whereby saidline will be a radial line and said difierent curve will be a secondspiral expanded with respect to said first spiral.

16. The system claimed in claim 10, comprising means for varying saidvoltage to control the beam for sweeping out a curve which is periodicalong the one Cartesian and second repetition rates are so selected thatthe dif- 10 ference between the ratio of one of said first and secondrates to said third rate and the ratio of the other of said first andsecond rates to said third rate is one.

No references cited.

CHESTER L. JUSTUS, Primary Examiner.

LEWIS H. MYERS, Examiner.

R. D. BENNETT, Assistant Examiner.

10. A RADIO RANGING SYSTEM COMPRISING A TRANSMITTER HAVING A KEYINGINPUT FOR CONTROLLING THE REPETITION RATE OF SIGNALS TRANSMITTED THEREBYTOWARD A TARGET; A RECEIVER FOR RECEIVING RESPONSE SIGNALS FROM THETARGET; MEANS FOR GENERATING A FIRST TRAIN OF KEYING PULSES AT A FIRSTRATE; MEANS FOR GENERATING A SECOND TRAINS OF KEYING PULSES AT A SECONDRATE; COINCIDENCE MEANS DERIVING FROM SAID FIRST AND SECOND PULSE TRAINSA THIRD PULSE TRAIN AT A THIRD RATE THE GREATEST COMMON DIVISOR OF SAIDFIRST AND SECOND RATES; SELECTOR LOGIC MEANS HAVING INPUTS CONNECTED TOSAID KEYING-PULSE-GENERATING MEANS AND OUTPUT CONNECTED TO SAID KEYINGINPUT OF THE TRANSMITTER; SAID LOGIC MEANS FURTHER HAVING A CONTROLLINGINPUT CONNECTED TO SAID COINCIDENCE MEANS WHEREBY TO APPLY SAID FIRSTAND SECOND KEYING-PULSE TRAINS TO SAID KEYING INPUT FOR RESPECTIVEPERIODS REGULARLY ALTERNATING IN ACCORDANCE WITH SAID THIRD RATE; ANOSCILLOGRAPHIC DISPLAY DEVICE INCLUDING ELECTRONBEAM-PRODUCING MEASN,DEFLECTION ELECTRODE MEANS, BEAM-CONTROL ELECTRODE MEANS AND A SCREEN;MEANS CONNECTING SAID RECEIVER TO SAID CONTROL ELECTRODE MEANS TO APPLYSAID RESPONSE SIGNALS THERETO FOR DISPLAY ONSAID SCREEN;VOLTAGE-DEVELOPING MEANS HAVING INPUTS CONNECTED TO SAID FIRSTKEYING-PULSE GENERATING MEANS AND SAID COINCIDENCE MEANS FOR DEVELOPINGAT LEAST ONE CYCLICALLY VARYING VOLTAGE HAVING A MINOR CYCLE PERIOD THERECIPROCAL OF SAID FIRST REPETITION RATE AND A MAJOR CYCLE PERIOD THERECIPROCAL OF SAID THIRD RATE; SAID VOLTAGE-DEVELOPING MEANS HAVING AVOLTAGE OUTPUT CONNECTED TO SAID DEFLECTION ELECTRODE MEANS OF THEDISPLAY DEVICE WHEREBY SAID ELECTRODE BEAM WILL SWEEP OUT ON THE SCREENA CYCLIC SCANNING CURVE HAVING A LINE REPETITION FREWUENCY EQUALING SAIDFIRST RATE AND A SCAN REPETITION FREQUENCY EQUALING SAID THIRD RATE, ANDWHEREBY SAID RESPONSE SIGNALS WILL BE DISPLAYED ON THE SCREEN AT THEINTERSECTIONS OF SAID SCANNING CURVE WITH A LINE INDICATIVE OF TARGETDISTANCE DURING PERIODS WHEN SAID FIRST KEYING PULSES ARE APPLIED AND ATTHE INTERSECTIONS OF SAID SCANNING CURVE WITH A DIFFERENT CURVE DURINGPERIODS WHEN SAID SECOND KEYING PULSE TRAIN IS APPLIED, AND THEINTERSECTION BETWEEN SAID LINE AND SAID DIFFERENT CURVE WILL PROVIDE ANUNAMBIGUOUS IDENTIFICATION OF TARGET DISTANCE.